Martian soil under the spotlight of new investigations
Space instrumentation laboratories at Université Paris- Saclay involved in the Mars 2020 and Exomars Mars exploration missions are set to carry out the first analysis of samples collected by the rovers.
The subject of many cinematic fantasies, Mars is above all an inexhaustible area of study for scientists all over the world. In February 2021, no less than three space missions which left Earth in July 2020 – in some of which the Atmospheres, Environments, Space Observations Laboratory (LATMOS – Université Paris-Saclay, CNRS, UVSQ) and the Institute of Space Astrophysics (IAS – Université Paris-Saclay, CNRS) were heavily involved – will reach the Red Planet. These include the Hope orbiter from the United Arab Emirates’ space agency, the Tianwen-1 probe, led by the Chinese National Space Administration (CNSA), and the Perseverance rover of the Mars 2020 mission led by the American space agency (NASA). The Rosalind Franklin rover, from the Exomars mission developed by the European (ESA) and Russian (Roscosmos) space agencies, was also due to leave in 2020, but its launch has been postponed until September 2022. Some researchers are investigating the planet’s soil, atmosphere, climate and glacial landscapes. Others – or the same scientists – are looking for traces of past life. All of them are trying to chart the history of this 4.6 billion year-old planet and, as a consequence, learn a little more about Earth.
“Even though the surface of Mars has been more or less frozen for three billion years, we now know that the planet’s evolution was similar to that of Earth over its first billion years and that its environment may have been suitable for the development of life,” explains Cyril Szopa from LATMOS. On Earth, this development involved liquid water, but also particular conditions of temperature, acidity or catalysts. “Finally, by studying this period on Mars, we also uncover things about the Earth’s past geology. Equivalent ancient rocks no longer exist on Earth as most of them have been recycled by plate tectonics.”
Mars has never experienced plate tectonics. However, just like Earth, in its early years the planet suffered numerous – and sometimes large – asteroid impacts which shaped its surface and topography. As a result, there is the contrast of a Northern hemisphere characterised by low-lying plains and a Southern hemisphere characterised by mountains. With its primitive atmosphere and magnetic field, the climate was hot and humid, and rivers formed (-4.1 to -3.7 billion years) in the North. Around 3.7 billion years ago, volcanic activity was at its peak. But for some as yet unclarified reason, Mars lost its magnetic field, which served as a shield. It then was subjected to the full force of the solar winds, which gradually and almost entirely eroded its atmosphere. The greenhouse effect disappeared and the planet cooled down. Fluvial activity ceased around 3.5 billion years ago. Large flooding valleys formed and irrigated a later ocean. Ice-rich permafrost formed in the northern plains and glaciers appeared in the southern highlands. From -3 billion years onwards, the temperature became incompatible with a sustained presence of liquid water on the surface and the planet began to die. With an average temperature of -65°C today, water only exists as ice.
Searching for molecules of life
Could life have been able to develop on Mars in its early years? To answer this question, scientists want to probe the rocks, looking for traces left by the passage of primordial liquid water, specific organic molecules – amino acids and sugars for example – and biosignatures, such as the presence of a single enantiomer. These are the objectives of the next missions which will set foot on Martian soil. “The instruments on board these rovers will study how the first stages of chemical evolution took place which, on Earth, led to the selection of compounds which resulted in biological living matter based, in particular, on DNA,” points out Jean-Pierre Bibring from the IAS. The Mars 2020 Perseverance rover will also collect samples which will be recovered by two further missions for a return to Earth by the end of the 2020s. This is a longawaited first.
The rovers’ landing sites are rich in sedimentary rocks (clays in particular), which are thought to have retained a record of conditions conducive to life. Jezero, the intended site for Perseverance, is an ancient delta which formed inside an impact crater. Similarly, sedimentary rocks found on Oxia Planum, where the Exomars 2022 rover will land, suggest that the site is an ancient river delta. “We know that deltas on Earth drain a lot of sediment and organic matter,” points out Cyril Szopa.
Space instrumentation for detection
IAS actively participated in the selection of the Mars 2020 and Exomars landing sites, and helped its Chinese counterparts to select the Tianwen-1 landing site. In collaboration with the Laboratory for Space Studies and Instrumentation in Astrophysics (LESIA – Observatoire de Paris, CNRS, Sorbonne Université, Université de Paris) and the Research Institute of Astrophysics and Planetology (IRAP – CNRS, CNES, Université Toulouse – Paul Sabatier) IAS has supervised the calibration of the IRS near-infrared point spectrometer of Supercam, the multilength and multi-scale instrument on board Perseverance. “Supercam comprises five instruments: a camera to capture the sample with a resolution of up to a few tens of micrometers, the IRS and Raman spectrometers for mineralogy, the LIBS spectrometer for elemental composition, and a microphone to reveal certain mechanical properties of the rock through the use of echoes,” explains François Poulet who is from IAS and is a member of the Mars 2020 team. Also involved in the design of Supercam, LATMOS has developed part of the electronics controlling this IRS infrared spectrometer, whose concept is inherited from the SPICAM instrument that has been flying over Mars since 2004 on board the Mars Express probe, and for which LATMOS has scientific and technical responsibility. A new adventure is awaiting this small spectrometer, but this time on the surface of the planet, where it will reveal the mineralogy and composition of the atmosphere at wavelengths as yet unexplored on Martian soil.
IAS, which was behind the OMEGA spectro- imager which left in 2003 with Mars Express, is an expert in infrared (IR) spectral imaging. This technique makes it possible to determine the physicochemical composition of samples on the basis of their optical properties. “Minerals have different colours under infrared wavelengths. By analysing the absorption spectrum obtained, it is possible to identify the compounds of the sample,” explains Jean- Pierre Bibring. “If liquid water was present in a stable way, the surrounding rocks are altered and salts, clays and sulphates can be detected.” As a result, OMEGA was the first to detect clays and sulphates in the Martian soil. The CRISM instrument from the Mars Reconnaissance Orbiter mission launched in 2005 confirmed OMEGA’s detection of these hydrated minerals and also detected carbonates. The IRS spectrometer on board Perseverance will be used to validate in situ the readings taken from orbit by OMEGA and CRISM.
IAS has also developed the MicrOmega visible- near-infrared hyperspectral microscope present on the Exomars rover. Just like Perseverance, Rosalind Franklin will house a space instrumentation laboratory in which the MicrOmega will work sequentially with two other instruments: the Raman RLS laser spectrometer and the gas chromatograph coupled to a MOMA (Mars Organic Molecule Analyser) mass spectrometer developed in part by LATMOS. Together, these instruments will be able to give a full analysis of the Martian soil samples (water and carbon content, identification of aliphatic and aromatic carbon phases and mineral properties, etc.).
“Inside MOMA, an oven heats the samples to a temperature of 900°C. The condensed matter vaporises into gas, releasing absorbed water molecules and organic and inorganic molecules, which give information about the mineral structure of the rock. These molecules are then separated by the chromatography systems and analysed by the mass spectrometer. We then obtain the molecular imprint of each of the molecules produced in the oven and we get back to the original organic molecule,” explains Cyril Szopa. MOMA is the worthy successor to the SAM instrument, also developed by LATMOS in collaboration with NASA’s Goddard Centre for the Mars Science Laboratory mission. Riding on board the Curiosity rover which landed on Mars in 2012, it was the first to reveal organic matter on the surface of Mars by detecting organic molecules containing chlorine and sulphur.
Probe for drilling
LATMOS is also behind the WISDOM ground-penetrating radar, present on the Exomars 2022 rover, which will probe the ground before drilling as it moves along. To find the organic molecules we are looking for, we must look in the Martian subsoil where it is sheltered from the ionising radiation which bombards the Martian surface and generates oxidation. While the rover drill is capable of digging to a maximum depth of 2 m, the objective is to know where to do it safely and to find interesting samples.
“The radar produces a 3D visualisation of the Martian subsoil,” says Valérie Ciarletti, scientific manager of the instrument at LATMOS. “Its antennas emit electromagnetic waves which penetrate the ground and reflect each time they encounter a change in the electrical properties of the environment.” It is therefore able to see the stratigraphy of the subsoil down to a depth of 3 m (10 m in an icy environment) with a resolution of a few centimetres. “Water has very special electromagnetic properties. The waves penetrate very well when it is in an icy state, but in a liquid or salty state, it behaves like a perfect reflector and the waves penetrate much less or not at all. The signal is then reduced or non-existent.” To get used to Martian conditions, the team carried out measurement operations in dry (Atacama desert) and frozen (Chamonix glaciers) environments, and in the cold room at the GEOPS laboratory. “With the help of the field data processing and interpretation programmes which have been developed, the presence of smooth or angular pebbles and the position of sediment layers to within 2-3 cm can be revealed.” This all has to take place in a short amount of time. “Within an hour, we must be able to tell where to drill or if we need to go somewhere else.”
Throughout the nominal duration of these missions (Perseverance should be operational for at least one Martian year, i.e. 687 Earth days, and Rosalind Franklin’s exploration should last 218 sols or Martian days, i.e. 224 Earth days), researchers will be ready to analyse the data collected by the instruments. The teams are now eagerly awaiting the first data.
• Melissa Guzman et al. Testing the capabilities of the Mars Organic Molecule Analyser (MOMA) chromatographic columns for the separation of organic compounds on Mars. Planetary and Space Science, 186, (2020).
• Y. Hervé et al. The WISDOM radar on board the ExoMars 2022 Rover: Characterization and calibration of the flight model. Planetary and Space Science, 189, (2020).
• Royer C. et al. Pre-launch radiometric calibration of the infrared spectrometer onboard SuperCam for the Mars 2020 rover. Rev Sci Instrum. 91, 6, (2020).
>>FOCUS: A hidden continent, glacial valleys and tsunami: Mars shakes up the codes
Recent discoveries made by researchers from the Paris- Saclay Geosciences laboratory (GEOPS – Université Paris- Saclay, CNRS) have provided new insights into the chronology of geological processes on Mars.
By artificially removing large impact craters, such as Hellas, and the volcanoes which formed during the first 500 million years, Sylvain Bouley and his colleagues are uncovering a hitherto hidden continent which is similar to the first land formed in the early history of Mars. This fine crustal block, about fifty kilometers thick, is located in the Terra Cimmeria Sirenum region in the southern hemisphere of Mars. “This proves that the contrast found on Mars between the thin crust in the North and the thick crust in the South, is the result of geological processes much more complex than initially thought,” explains Sylvain Bouley. But how did this block form? This remains a mystery. “However, by understanding the formation of the continents on Mars, we can explain what first occurred on Earth, where plate tectonics only appeared 3 billion years ago.”
Planetologists count the number of impact craters to date the land and trace the chronology of events. “Thanks to the lunar samples brought back to Earth by the American Apollo missions and absolute dating, a rule has been established linking the density of craters on the surface to the moon’s age. This law was applied to Mars and it suggested that its cratering rate had been strong around 4 billion years ago, then continuously decreased to become constant from 3 billion years ago. In the laboratory, we have recently shown that this rate of cratering is not as constant as first imagined.” The morphology of the craters –with lobate or grooved ejecta – and the mineralogical analysis of these ejecta, which bring buried material to the surface, also provide valuable information. “As a result, it has been possible to date river and volcanic activity on Mars. Water would have flowed between 3.8 and 3.5 billion years ago and the Tharsis Dome – a large volcanic feature 5,000 km in diameter located in the northern hemisphere – would have formed at the same time as networks of underground rivers. We have also shown that these rivers were distributed over a tropical band similar to the equator before Mars tilted on its axis and reoriented itself as a result of the dome being pushed up.”
The change in Mars’ obliquity would also have led to significant climate change on the planet. “We have studied valleys located at very high altitudes in the southern hemisphere dating back to 3.6 billion years ago and found them to be similar to the U-shaped glacial valleys on Earth. This means that there would have been a cold climate earlier than expected,” says Antoine Séjourné.
Curvilinear ridges located in a region of Terra Arabia (a compact zone between the low plains of the North and the high plateaus of the South) and re-studied 30 years after the images taken by the Viking probe using high-resolution images from Mars Express and topographic data, have revealed to François Costard and his colleagues lobed formations which rised from the slopes over 100 m. “We have concluded that these are mud deposits caused by a tsunami. This means that there was a liquid ocean there, which we have dated to 3 billion years ago,” explains François Costard. These results call into question the idea that all oceans would have been frozen after 3.5 billion years ago. “On Earth, a tsunami is usually caused by seismic tremors. However, on a one-plate planet like Mars, it would instead be the result of a meteorite impact.” After pinpointing about ten large impact sites in the northern low plains north of Arabia Terra as possible starting points for this tsunami, researchers applied a digital propagation model for tsunami initially computed for Earth and adjusted to Mars. In the end, the only one capable of generating a tsunami which could have covered shores with lobed formations is the Lomonosov crater, with a diameter of 150 km. “It would have generated two successive waves, each 300 m high, which would have travelled at the speed of a TGV train.” Using a morphometric approach, they found that this crater showed signs of being an ancient marine crater with wide collapsed ramparts, and also dated it to 3 billion years ago. A further facet of the Lomonosov crater was discovered. They also revealed accompanying volcanic activity located in an area of lobed deposits. Mud cones line the slopes formed by the compaction of fine waterlogged sediments drained by the tsunami.
• Bouley S. et al. A thick crustal block revealed by reconstructions of early Mars highlands. Nat. Geosci. 13, (2020).
• Bouquety, A. et al. Glacial landscape and paleoglaciation in Terra Sabaea: Evidence for a 3.6 Ga polythermal plateau ice cap. Geomorphology, 350, (2020).
• Ilaria Di Pietro et al. Evidence of mud volcanism due to the rapid compaction of martian tsunami deposits in southeastern Acidalia Planitia, Mars. Icarus, 354, (2021).